HIV-1 integrase is essential for the HIV replication cycle, furthermore it is a key enzyme for the ability of the HIV virus to infect non-dividing cells. In addition to reverse transcriptase and protease, integrase has also been the focus of attention for HIV antiviral chemotherapy. Integrase is an attractive target because it has no counterpart in mammalian cells; therefore, selective integrase inhibitors should not produce any side effects. Many integrase inhibitors have been reported to date. However, progress with the design of effective inhibitors of HIV integrase is slower than in the case of reverse transcriptase or protease inhibitors and no clinically useful drugs have yet been approved. Our project is focused on specific peptide sequences with HIV-1 integrase inhibitory activity. For one of these sequences (HCKFWW) we designed and synthesized several analogs (including cyclic and dimeric peptides) for structure-activity studies. We found that dimeric analogs of this peptide (dimerized through the Cys residue side-chains or C-ends of the monomers) are distinctly more potent than the monomer. The inhibitory potency of dimers depends on linker length and composition and it is up to 20 times more potent than the monomer. Based on these results we synthesized the dimers of other known integrase inhibitory peptides (usually dimerized at C-ends, using lysine as a linker). We also designed and synthesized tetramers of these peptides in which the C-ends of the sequences were connected to four amino groups of the template build from 3 residues of lysine. For all sequences the dimer was more potent than the monomer and the tetramer was the most potent inhibitor (up to over 200 times more potent than monomer). We also tested the influence of the template composition on the inhibitory potency using templates containing ornithine or 2,4 diaminobutanoic acid. The results were similar to the results for the template containing lysine. But for the dimers with a more rigid linker (3,5-diaminobenzoic acid) the inhibitory potency was found to be slightly lower ( 2 times). We also tested some natural product peptides and we discovered HIV-1 integrase inhibitory activity of indolicidin, a potent antimicrobial agent, with a tryptophan rich 13 mer peptide sequence - ILPWKWPWWPWRR-NH2, which was isolated from the cytoplasmic granules of bovine neutrophils in 1992). The tetramer of indolicidin Pro-Pro analog (with sequence ILPWKWPWWPWPP) is one of the most active integrase inhibitory peptides (IC50 = 0.7 and 0.4 M, for 3' processing and strand transfer, respectively). The preliminary results we have for some of the tetramers indicate the IC50 values lower than 150 nM. With these values of IC50, the tetrameric peptides, as far as we know, are the most potent HIV-1 integrase inhibitory peptides. The high inhibitory potency of the tetrameric peptides suggests that these peptides may interact with multiple active centers (2 or 4) within the multimeric complex of integrase. This is consistent with known fact that tetrameric or octameric complex of integrase is required to accomplish the integration process.